Sinnoh is definitely the best for me, even though the games weren't my favourite, the region itself was just superior. I managed to clock 140+ hours on Platinum which suggests there is a lot to do within the region as well.

Kanto and Johto were similarly great, Hoenn was pretty cool and although I like Unova , it was just like a reject version of Sinnoh really (much like the games themselves due to the lack of console progression between the gens).